Co-evolution of microstructure and dislocation dynamics in InGaP/GaP : engineering high quality epitaxial transparent substrates

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000.Includes bibliographical references (p. 253-261).Graded buffers oflnxGa1-xP on GaP ([Delta]x[lnxGa1-x]P/GaP) can be used to fabricate potentially high performance, epitaxial transparent substrates light-emitting diodes (ETSLEDs ). Practical devices have thus far been limited by poor quality: reports of [Delta]x[lnxGa1-x]P/GaP show sharp declines in device and material quality above x - 0.3. This study revisits the challenge of engineering high-quality [Delta]x[lnxGa1-x]P/GaP grown by metal-organic vapor phase epitaxy. A new planar defect microstructure oriented 10-15° off the (1-10), which we call branch defects, was discovered via transmission electron microscopy. Branch defects feature sharp strain fields and dominate the microstructure. causing dislocation pinning and escalation. Branch defects occur later in growth with increasing temperature; however, they are stronger when formed at higher temperatures. Branch defects do not appear to be directly related to other co-existing microstructures in lnxGa1.xP. In the phase space where branch defects are absent, the intrinsic dislocation dynamics of[Delta]x[lnxGa1-x]P/GaP were explored. Dislocation density decreases exponentially with increasing temperature, supporting a kinetic glide model for graded buffers. Dislocation glide velocities also appear to increase dramatically while grading from GaP to InP. Optimizing the co-evolution of dislocation dynamics and branch defects has achieved dislocation densities of [Delta]x106 cm-2 out to x = 0.54, the highest quality [Delta]x[lnxGa1-x]P/GaP reported to date. Reciprocal space mapping reveals three distinct regimes of crystallographic tilt. Qualitative to semi-quantitative models were developed for each regime to elucidate the changing dislocation dynamics during V x[lnxGai-x]P/GaP growth. Critical reanalysis of earlier reports provides further evidence for the kinetic glide model. Overall, discovery of tilt regimes demonstrates the need for a dynamic approach to tilt analysis. A series of ETS-LEDs with emission wavelengths ranging from 575 to 655 nm was fabricated from optimized [Delta]x[lnxGa1-x]P/GaP and shows continuing good performance for [Delta]x 0.3, in contrast to earlier reports. A second, subtle process optimization to better suppress branch defects increases efficiency 60% and drops spectral width 8 meV. Since self-absorption in [Delta]x [lnxGai-x]P/G90%, a fully transparent [Delta]x,y [inx(AlyGa1_y)i-x]P/GaP technology was also developed and initial results promise an order of magnitude improvement in device efficiency. The improvements from subtle process changes suggest a good outlook for achieving practical ETS-LEDs.by Andrew Y. Kim.Ph.D